Adding support for test signal in Template.stream method

Jinja2 has a great feature for streaming a template rendering [1]. To use this in Django without losing the `TestCase.assertTemplateUsed` I've implemented the `Template.stream` just like the current `Template.render` and as a bonus added tests for them.

If there is any improvement needed, please, let me know. :smile: 

[1] http://jinja.pocoo.org/docs/api/#jinja2.Template.stream